1. Field of the Invention
The present invention relates to data storage devices. More particularly, the present invention relates to rotating disk magnetic data storage devices.
2. Description of the Prior Art
Rotating disk magnetic storage devices, commonly referred to as "disk drives", have undergone dramatic improvements in physical size versus data storage capacity in recent years. For example, disk drives having data storage capacities of 40 to 80 megabytes in a package of less than 10 cubic inches are not uncommon. These physically smaller disk drives usually have correspondingly reduced power requirements.
Similarly, portable computers have also undergone dramatic improvements in recent years so that high-performance, battery-powered portable computers are now readily available. Thus, a modern portable computer weighing only a few pounds has most of the storage, display, and computing capabilities of a desk-top model. As a result, modern portable computers require relatively large data storage capacities in an environment limited both in physical size and available electrical power.
Accordingly, for portable computers which use batteries to provide electrical power, continuous operation of a disk drive may be power intensive and result in limited operating time between battery replacement or recharging. The constraints imposed on portable computers have thus created a significant need for a large capacity disk drive having a relatively low power consumption.
The power consuming components of a disk drive may be generally divided into analog and digital sections. The digital section is related to disk and read/write control functions. The analog sections are the data read/write processing section and the motor control sections. The most power intensive sections are those related to motor control; i.e. the actuator motor which drives the supporting actuator for the read/write transducer heads, the spindle drive system which controls the spindle motor which rotates the magnetic data storage disk, and the associated drive electronics. In particular, the spindle drive system and spindle motor require significant power both to overcome stiction in order to initially start the magnetic storage disk, accelerate to the desired operating speed, and to rotate the magnetic storage disk at a constant velocity in order to "fly" the read/write transducer during data read/write and track following operations.
The ability of the spindle drive system to meet the power consumption requirements of portable computers is directly affected by spindle motor size, winding energization mode and motor efficiency. The efficiency factor or "figure of merit" ("K.sub.M ") of the spindle motor is related to the physical size and design efficiency of the motor, and is proportional to the torque squared divided by the power dissipated within the windings. The torque, in turn, is proportional to the driving current of the windings. Since the power is proportional to the current squared, the K.sub.M is a constant for a given spindle motor design and its driving mode.
Conventionally, the motor design and driving mode is selected based upon two principal criteria: (1) the amount of stall torque first needed to overcome both the disk drive's stiction torque to effect relative motion of the read/write transducer heads and the cogging and bearing friction torque of the spindle; and (2) the amount of continuous torque then needed to rotate the magnetic storage disk at a constant velocity during data read/write and track following operations. In general, large disk drives have larger torque requirements than smaller disk drives. However, the amount of start torque needed by both large and small disk drives is typically large enough to require a considerable amount of drive current during startup, with considerably less drive current required once the target rotational velocity has been reached.
The number of disk start/stops versus runs, along with the corresponding start versus run times, also affects the overall power consumption. Starting the spindle motor requires relatively high torque for a short period of time, and also draws a relatively high current. Additionally, a disk drive adapted for use with a portable computer may be allowed to spin down after a short period of inactivity in order to conserve power. Thus, the frequency of start/stops of a spindle motor is much higher for the smaller disk drives. However, each time data is accessed after the drive is spun down, the high torque, high power start must be initiated. Therefore, repeated high power starts may offset any reduced power consumption achieved through spin down in a low power mode.
Accordingly, there is a need for a compact spindle drive system which is capable of starting a spindle motor, running the motor at a desired rotational velocity, and stopping the motor in a manner which provides an overall reduced power consumption.